[CMake] Refactor subdirectory inclusion code to take a project name.

Summary:
This refactoring makes some of the code used to control including subdirectories parameterized so it can be re-used elsewhere.

Specifically I want to re-use this code in clang to be able to turn off specific tool subdirectories.

Reviewers: chapuni, filcab, bogner, Bigcheese

Subscribers: emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D13783

llvm-svn: 250835